A covalent bond is a chemical bond that forms when valence electrons are shared between atoms. Nonpolar covalent bonds form between two identical elements or between elements with identical affinities for electrons. A nonpolar covalent bond is a covalent bond in which the nuclei of the bonded atoms exert equal or nearly equal force on the shared electrons. For example, H2 contains nonpolar covalent bonds because the hydrogen (H) atoms are identical and they attract their shared electrons with identical strength. The tendency of an atom to attract electrons toward itself when forming bonds is its electronegativity.Atoms with different electronegativities form polar covalent bonds. A polar covalent bond is a covalent bond in which the electron density is more localized on one end of the bond. One end is slightly positive, and one end is slightly negative. In these molecules, the shared electrons in the covalent bond pull toward the atom with the higher electronegativity. For example, hydrogen fluoride has a polar covalent bond because fluorine is more electronegative than hydrogen, so it pulls the electrons toward itself, and the distribution of electrons in the bond is polarized. A covalent bond becomes more polar as the difference between the electronegativities of its atoms increases. Generally, electronegativity increases from left to right across the periodic table and decreases down the columns. Fluorine is the most electronegative element, followed by oxygen.
Selected Bond Dipole Moments of Hydrogen
Selected Bond Dipole Moments of Carbon